用 C 处理位流

发布于 2024-11-16 08:43:08 字数 396 浏览 2 评论 0原文

我正在用 C 语言编写一些具有通信通道的信号处理代码。在输出端，当它们到达时我得到了一堆位。

for (n=0; n<BUFFER_LENGTH; n++) {
    /* do some processing that calculates x */
    output[n] = x > 0;
}

这是我的问题：

有没有一个好的类型可以代表输出数组？起初我以为 uint1_t 是理想的选择，但我听说这并不一定代表内存中的一点点。
一旦我找到一个我知道的数据中的同步模式接下来的位的格式，我怎样才能将一堆 1 和 0 转换为将数组转换为整数、浮点数、双打、角色等？我听说过使用一个工会，但我认为这不会与位数组。

原文

I am writing some signal processing code in C that has a communications channel. At the output I get a bunch of bits as they arrive.

for (n=0; n<BUFFER_LENGTH; n++) {
    /* do some processing that calculates x */
    output[n] = x > 0;
}

Here are my questions:

Is there a good type to represent
the output array? At first I thought
uint1_t would be ideal but I hear
that doesn't necessarily represent
one bit in memory.
Once I find a
sync pattern in the data I know the
format of the next bits, how can I
convert a bunch of 1's and 0's in
the array into integers, floats,
doubles, characters, etc.? I've heard of using
a union but I don't think that will work with an
array of bits.

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秋千易 2024-11-23 08:43:09

只需将数据存储在足够大的字节块中，然后使用移位和掩码迭代这些位，以顺序提取各个位。

例如，将缓冲区的内容打印为单独的位：

#include <stdio.h>
#include <stdint.h> // uint8_t et al
#include <limits.h> // CHAR_BIT

uint8_t buffer[256];
int b, bit;

for (b = 0; b < 256; ++b)
{
    for (bit = CHAR_BIT - 1; bit >= 0; --bit)
    {
        uint8_t mask = 1 << bit;
        printf("%2d", (buffer[b] & mask) != 0);
    }
    printf("\n");
}

Just store the data in a sufficiently large block of bytes and then iterate through the bits using shifting and masking to extract individual bits sequentially.

e.g. to print out the contents of a buffer as individual bits:

#include <stdio.h>
#include <stdint.h> // uint8_t et al
#include <limits.h> // CHAR_BIT

uint8_t buffer[256];
int b, bit;

for (b = 0; b < 256; ++b)
{
    for (bit = CHAR_BIT - 1; bit >= 0; --bit)
    {
        uint8_t mask = 1 << bit;
        printf("%2d", (buffer[b] & mask) != 0);
    }
    printf("\n");
}

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孤独陪着我 2024-11-23 08:43:09

您必须以 8 位的倍数来处理它们，并自由地使用掩码（& 运算符）和移位（>> 运算符）。
因此，以 MP3 比特流为例（来自 http://www.mp3-tech .org/programmer/frame_header.html)：

AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM

它可以保存在一个 32 位变量或四个 8 位变量中，如下所示：

MMLKJJII HGFFEEEE DCCBBAAA AAAAAAAA

这里你有：

int i = 0;
while (!syncFound)
{
    uint16 sync = data[i] | ((data[i + 1] & 0x07) << 8);
    if (sync == 0x07ff)
    {
        // The first 11 bits represent a sync.
        uint8 version = (data[i + 1] & 0x18) >> 3;
        uint6 layer = (data[i + 1] & 0x60) >> 5);
        // etc...

        syncFound = TRUE;
    } else {
        // current byte is not the start of a frame. check if the next byte is.
        i++;
    }

}

数组边界检查可能会变得乏味，因此也可能会变得乏味交易具有可变长度标头。

You'll have to deal with them in multiples of 8 bits and use masking (& operator) and shifting (>> operator) liberally.
So, to take the MP3 bitstream as an example (from http://www.mp3-tech.org/programmer/frame_header.html):

AAAAAAAA AAABBCCD EEEEFFGH IIJJKLMM

which can be held in either one 32-bit variable or four 8-bit ones as:

MMLKJJII HGFFEEEE DCCBBAAA AAAAAAAA

Here you'd have:

int i = 0;
while (!syncFound)
{
    uint16 sync = data[i] | ((data[i + 1] & 0x07) << 8);
    if (sync == 0x07ff)
    {
        // The first 11 bits represent a sync.
        uint8 version = (data[i + 1] & 0x18) >> 3;
        uint6 layer = (data[i + 1] & 0x60) >> 5);
        // etc...

        syncFound = TRUE;
    } else {
        // current byte is not the start of a frame. check if the next byte is.
        i++;
    }

}

Array bounds checking can get tedious and so too can dealing with variable length headers.

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